1. Field
The presently disclosed subject matter relates to light emitting devices using at least one LED, and more particularly to a reliable light emitting device having a countermeasure against static electricity with the possibility of being manufactured in a small size, and which can be configured to emit light in parallel with a mounting surface thereof (e.g. a side view type LED device).
2. Description of the Related Art
Electronic devices have recently become thinner and generally downsized in keeping with the trend of miniaturization in electronic products such as a mobile phones, personal computers, audio players, etc. When these electronic devices are mounted on a circuit board, light emitting devices have become a relatively high-height part in comparison with other electronic devices.
Therefore, when static electricity is generated around the circuit board, static electricity may be attracted to the high-height light emitting device. If the light emitting device has a low electrostatic breakdown voltage, the light emitting device may be damaged by static electricity and the electronic products that include the light emitting device may also become defective.
As a method for preventing a light emitting device from damage caused by a forward overvoltage and/or a reverse overvoltage that may be generated by static electricity, an LED device including a protective element is disclosed in Patent Document No. 1 (Japanese Patent Application Laid Open H11-054804). FIG. 8(A) is a cross-section view depicting a conventional LED device disclosed in Patent Document No. 1.
The conventional LED device includes: a circuit board that is made by forming conductor patterns 11, 12 on surfaces of an insulating board 10; an LED chip 13 mounted on the conductor pattern 11; bonding wires 14 for connecting electrodes of the LED chip 13 to the conductor patterns 11, 12; a zener diode 15 mounted on the conductor pattern 12 and electrically connected to the conductor patterns 11, 12 in parallel with the LED chip 13; and an encapsulating resin 16 disposed in a cavity 19 formed on the circuit board.
FIG. 8(B) is a circuit diagram showing a relation between the LED chip 13 and the zener diode 15 that is used as the protective element. The zener diode 15 can have current that flows not only in the forward direction like a normal diode but also in the reverse direction when the applied voltage is larger than the zener voltage. Thus, the zener diode 15 can prevent the LED device 13 from damage caused by both forward overvoltage and reverse overvoltage that may be generated by static electricity.
However, the conventional LED device includes certain features and problems, such as an increase of the cost for the protective element, an increase in steps of the manufacturing process for mounting the protective element, an enlargement of the space, etc. In addition, the protective element may cause a negative effect in light distribution characteristics of the LED device.
Therefore, a protective device for an LED device without the above-referenced protective element is disclosed in Patent Document No. 2 (Japanese Patent Application Laid Open JP2001-196638). FIG. 9(A) is a partial top view depicting a protective device for an LED device disclosed in Patent Document No. 2.
The protective device includes: a circuit board 24 being formed a conductor pattern 23 on a surface of an insulating board 22; an LED device 21 mounted on the circuit board 24 by connecting lead wires 21a thereof to conductor patterns of the circuit board 21; and an electric conductor 25 electrically connected to the conductor pattern 23 of the circuit board 24 so as to surround the LED device 21 therein.
In this case, the electric conductor 25 can maintain an electric potential of ground because the conductor pattern 23 is connected to ground. Thus, because the surface around the LED device 21 located in the electric conductor 25 may maintain an electric potential of ground even if static electricity is generated around the circuit board 24, the protective device can prevent the LED device 21 from problems caused by static electricity.
The conventional protective device may result in the mounting portion of the LED device 21 being relatively huge because the mounting height must become higher than both heights of the circuit board 23 and the LED device 21, and the additional round conductor is also located on the circuit board 24. In addition, the surface underneath the LED device 21 may cause a negative effect such as an unnecessary reflection in light distribution characteristics for the LED device 21.
FIG. 9(B) is a partial top view depicting a variation in the protective device shown in FIG. 9(A). In the protective device shown in FIG. 9(B), an electric conductor 26 is formed in a U-shape and the surface under the LED device 21 is eliminated. Therefore, the negative effects in the protective device shown in FIG. 9(A) may be reduced.
However, because the above-described protective devices include electric conductors 25, 26 on the circuit board 22, the LED device 21 may be limited to certain locations on the circuit board 22. Moreover, because the LED device 21 may be difficult to mount on a surface of the circuit board 22, it may be difficult to make the mounting portion thin and the mounting structure may become a weak structure relative to shock, vibration, etc.
The above-referenced Patent Documents are listed below, and are hereby incorporated with their English abstracts in their entireties.    1. Patent Document No. 1: Japanese Patent Application Laid Open H11-054804    2. Patent Document No. 2: Japanese Patent Application Laid Open JP2001-196638
The disclosed subject matter has been devised to consider the above and other problems and characteristics. Thus, according to an aspect of the disclosed subject matter a reliable light emitting device can include a countermeasure against static electricity and can be mounted on a surface of a circuit board and can be miniaturized.